The present invention is directed internal bicycle hub transmissions and, more particularly, to an internal hub transmission with a clutch that rotates and moves axially.
Bicycles, particularly recreational bicycles referred to as city cruisers, are inexpensive and are easy to ride, and are thus widely used to commute to work or school or for shopping. With this type of recreational bicycle, an internal hub transmission is sometimes mounted at the rear wheel in order to ride at high speeds over flat terrain or to ride uphill with minimal exertion.
An internal hub transmission generally comprises a hub axle that is fixed to the bicycle frame; a hub shell that is able to rotate around the hub axle; a planet gear mechanism that is housed in the hub shell; a clutch mechanism for selecting among a plurality of drive transmission paths with an interposed planet gear mechanism; and a bell crank for moving the clutch member. The clutch mechanism has a clutch member for switching the drive transmission path by moving in the direction of the hub axis, and a push rod that presses the clutch member. The bell crank is linked via a shift cable to a shift lever mounted to the handlebar, for example, in order to control shifts.
With a typical internal hub transmission, operation of the shift lever winds and pulls the inner cable of the shift cable, thus causing the bell crank to push on the push rod and move the clutch member in one axial direction to perform a gear shift from a higher to a lower speed step (downshifting). However, when the inner cable is slackened to perform a gear shift from a lower to a higher speed step (upshifting), the clutch member will not move in the opposite direction by movement of the push rod alone. Therefore, the biasing force of a return spring or the like is normally used to assist movement of the clutch member in the other axial direction.
With the conventional internal hub transmission described above, a large amount of resistance is generated between the clutch member and the parts that make up the planet gear mechanism when a large drive force is applied. More specifically, when a large drive force is applied, frictional force increases in the engaged portions between the clutch member and the parts that make up the planet gear mechanism, and this makes it more difficult for the gears to be shifted by movement of the clutch member. In order to overcome this additional frictional force, the inner cable of the control cable must be pulled with even greater force when downshifting. Also, the spring force of the return spring must be large enough to overcome the frictional force when the inner cable is slackened when upshifting. If the spring force of the return spring is increased, however, then the inner cable of the control wire must be pulled with still greater force when downshifting. Consequently, in order to allow the gears to be shifted when a large drive force is applied during riding regardless of the direction of the shift, great strength is needed to operate the shift lever. This is, very undesirable as it makes shifting very fatiguing if not impossible for some riders.